Great whites on the prowl in Cape Cod waters

Thursday

Jul 25, 2013 at 12:01 AMJul 25, 2013 at 1:57 AM

You’re not on the menu.

John Chisholm has been fielding calls from the media for weeks now, thanks to great white shark sightings everywhere from Cape Cod Bay to the coasts of Chatham, Truro and Nantucket. These prehistoric behemoths feed on seals, and that’s what draws them here.

They are not here because they crave 45-year-old accountants, 20-year-old interns or any other humans, for that matter, he said.

Emily Clark

You’re not on the menu.

John Chisholm has been fielding calls from the media for weeks now, thanks to great white shark sightings everywhere from Cape Cod Bay to the coasts of Chatham, Truro and Nantucket. These prehistoric behemoths feed on seals, and that’s what draws them here.

They are not here because they crave 45-year-old accountants, 20-year-old interns or any other humans, for that matter, he said.

Great whites have also always been here, according to Chisholm, but they don’t find us tasty.

A Marine Fisheries biologist and Massachusetts Shark Research Program scientist, one of Chisholm’s many jobs is implementing and monitoring shark tagging so he and his crew can log important data on great whites and other sharks in the area. Working with his boss, Division of Marine Fisheries Senior Biologist Greg Skomal, Chisholm said one of the challenges is educating the public about these massive creatures so that full-scale panic doesn’t erupt when a sighting is reported.

“They don’t want to eat people,” Chisholm said. “We’re way too bony for them. They know what they want to eat and it’s not us.”

While great whites are responsible for one-third to one-half of all shark attacks worldwide, most aren’t fatal. A great white or other carnivorous shark may take a sample or test bite if they’re not quite sure what they’re dealing with. But with their massive jaws and razor sharp teeth that one bite can, and sometimes does, kill or maim.

But great whites aren’t the only sharks making the rounds in these waters. Makos, sand tigers, threshers, spiny dogfish and blue sharks have been prowling Cape Cod Bay, Nantucket Sound and Buzzards Bay plus points north and south for time out of mind. More than 90 percent of great white sightings are actually basking sharks, Chisholm explained, which can reach up to 37 feet in length and like to hover near the surface. Great whites don’t behave that way, unlike their portrayal in the movie Jaws, Chisholm added. That dorsal fin sliding along the surface is typical of the basking shark; great whites like to surprise their victims by coming at them from below, not head on.

The tagging program is important because it tracks these shark’s movements, which extend from Canadian waters to New England and all the way to Florida during a given year. An online site dubbed “Osearch” shows the location of these tagged sharks at any given time. Each tagged shark is named and anyone can visit the site, scroll to the particular shark they’re interested in and see where it is.

Not much is known about these animals, and Chisholm’s work is all about filling in those blanks.

The process involves first tagging the animal, usually via a motorboat or fishing vessel. A harpoon-like device is used to shoot the shark with the tag, in some cases, but in others, the shark is caught with a hook and dragged aboard where a hose is inserted into its mouth to keep water continually circulating through its gills. Tags are then clipped onto fins before the shark is released back into the ocean.

Different tags do different things. For instance, one logs in the site of the shark’s breech, when it surfaces briefly, while another tracks its movement underwater. Some also record temperature and other readings that help scientists identify the shark’s behavior.

Great whites can reach 20-plus feet in length, can weigh up to 5,000 pounds and can live for 30 years or more. They tend to travel solo, have several rows of up to 300 serrated, triangular teeth and have organs that detect tiny electromagnetic fields generated by animals in the water. According to experts, the great white shark species is about 16 million years old.

Bull sharks are the most dangerous sharks to humans, Chisholm said.

“They’ve been found in the Mississippi River and all the way up to St. Louis. They like areas where the water is turbid and cloudy,” he added.

The key to avoiding a shark attack is to stay away from where they feed, according to Chisholm, who noted that bull sharks and several other types of sharks like to lurk in the mouths of rivers at dawn and dusk. White sharks want to eat seals, and Chisholm advised swimmers to avoid swimming anywhere near seals.